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Figure 8 Positions of Layer 3 multicast protocols
• Multicast group management protocols:
IGMP and MLD protocol are multicast group management protocols. Typically, they run between
hosts and Layer 3 multicast devices that directly connect to the hosts to establish and maintain
multicast group memberships.
• Multicast routing protocols:
A multicast routing protocol runs on Layer 3 multicast devices to establish and maintain multicast
routes and correctly and efficiently forward multicast packets. Multicast routes constitute loop-free
data transmission paths (also known as multicast distribution trees) from a data source to multiple
receivers.
In the ASM model, multicast routes include intra-domain routes and inter-domain routes.
{ An intra-domain multicast routing protocol discovers multicast sources and builds multicast
distribution trees within an AS to deliver multicast data to receivers. Among a variety of mature
intra-domain multicast routing protocols, PIM is most widely used. Based on the forwarding
mechanism, PIM has dense mode (often referred to as "PIM-DM") and sparse mode (often
referred to as "PIM-SM").
{ An inter-domain multicast routing protocol is used for delivering multicast information between
two ASs. So far, mature solutions include Multicast Source Discovery Protocol (MSDP) and
MBGP. MSDP propagates multicast source information among different ASs. MBGP is an
extension of the MP-BGP for exchanging multicast routing information among different ASs.
For the SSM model, multicast routes are not divided into intra-domain routes and inter-domain
routes. Because receivers know the position of the multicast source, channels established through
PIM-SM are sufficient for the transport of multicast information.
Layer 2 multicast protocols
Layer 2 multicast protocols include IGMP snooping and MLD snooping.
IGMP snooping and MLD snooping are multicast constraining mechanisms that run on Layer 2 devices.
They manage and control multicast groups by monitoring and analyzing IGMP or MLD messages
exchanged between the hosts and Layer 3 multicast devices. This effectively controls the flooding of
multicast data in Layer 2 networks.
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